Greg Holland

Solubility trapping in formation water as dominant CO2 sink in natural gas fields

Injecting CO2 into deep geological strata is proposed as a safe and economically favourable means of storing CO2 captured from industrial point sources. It is difficult, however, to assess the long-term consequences of CO2 flooding in the subsurface from decadal observations of existing disposal sites. Both the site design and long-term safety modelling critically depend on how and where CO2 will be stored in the site over its lifetime. Within a geological storage site, the injected CO2 can dissolve in solution or precipitate as carbonate minerals. Here we identify and quantify the principal mechanism of CO2 fluid phase removal in nine natural gas fields in North America, China and Europe, using noble gas and carbon isotope tracers. The natural gas fields investigated in our study are dominated by a CO2 phase and provide a natural analogue for assessing the geological storage of anthropogenic CO2 over millennial timescales. We find that in seven gas fields with siliciclastic or carbonate-dominated reservoir lithologies, dissolution in formation water at a pH of 5–5.8 is the sole major sink for CO2. In two fields with siliciclastic reservoir lithologies, some CO2 loss through precipitation as carbonate minerals cannot be ruled out, but can account for a maximum of 18 per cent of the loss of emplaced CO2. In view of our findings that geological mineral fixation is a minor CO2 trapping mechanism in natural gas fields, we suggest that long-term anthropogenic CO2 storage models in similar geological systems should focus on the potential mobility of CO2 dissolved in water.

Seawater subduction controls the heavy noble gas composition of the mantle.

The relationship between solar volatiles and those now in the Earths atmosphere and mantle reservoirs provides insight into the processes controlling the acquisition of volatiles during planetary accretion and their subsequent evolution. Whereas the light noble gases (helium and neon) in the Earths mantle preserve a solar-like isotopic composition, heavy noble gases (argon, krypton and xenon) have an isotopic composition very similar to that of the modern atmosphere, with radiogenic and (in the case of xenon) solar contributions. Mantle noble gases in a magmatic CO2 natural gas field have been previously corrected for shallow atmosphere/groundwater and crustal additions. Here we analyse new data from this field and show that the elemental composition of non-radiogenic heavy noble gases in the mantle is remarkably similar to that of sea water. We challenge the popular concept of a noble gas ‘subduction barrier’—the convecting mantle noble gas isotopic and elemental composition is explained by subduction of sediment and seawater-dominated pore fluids. This accounts for ∼100% of the non-radiogenic argon and krypton and 80% of the xenon. Approximately 50% of the convecting mantle water concentration can then be explained by this mechanism. Enhanced recycling of subducted material to the mantle plume source region then accounts for the lower ratio of radiogenic to non-radiogenic heavy noble gas isotopes and higher water content of plume-derived basalts.

Deep fracture fluids isolated in the crust since the Precambrian era

Fluids trapped as inclusions within minerals can be billions of years old and preserve a record of the fluid chemistry and environment at the time of mineralization. Aqueous fluids that have had a similar residence time at mineral interfaces and in fractures (fracture fluids) have not been previously identified. Expulsion of fracture fluids from basement systems with low connectivity occurs through deformation and fracturing of the brittle crust. The fractal nature of this process must, at some scale, preserve pockets of interconnected fluid from the earliest crustal history. In one such system, 2.8 kilometres below the surface in a South African gold mine, extant chemoautotrophic microbes have been identified in fluids isolated from the photosphere on timescales of tens of millions of years. Deep fracture fluids with similar chemistry have been found in a mine in the Timmins, Ontario, area of the Canadian Precambrian Shield. Here we show that excesses of 124Xe, 126Xe and 128Xe in the Timmins mine fluids can be linked to xenon isotope changes in the ancient atmosphere and used to calculate a minimum mean residence time for this fluid of about 1.5 billion years. Further evidence of an ancient fluid system is found in 129Xe excesses that, owing to the absence of any identifiable mantle input, are probably sourced in sediments and extracted by fluid migration processes operating during or shortly after mineralization at around 2.64 billion years ago. We also provide closed-system radiogenic noble-gas (4He, 21Ne, 40Ar, 136Xe) residence times. Together, the different noble gases show that ancient pockets of water can survive the crustal fracturing process and remain in the crust for billions of years.

Meteorite Kr in Earth’s Mantle Suggests a Late Accretionary Source for the Atmosphere

Gas Leak Inspection The solid portion of Earth was formed from accretion of material and debris formed in the primitive Solar System. Earths early evolution included the differentiation of its interior and the development of a primordial atmosphere. Heavy noble gases in the atmosphere could have been acquired during the initial accretion process or may have accumulated later through gravitational volatile capture. Holland et al. (p. 1522) show that Kr and Xe trapped in the upper mantle have isotopic signatures characteristic of early Solar System material similar to meteorites rather than those of the modern atmosphere and oceans. Thus, noble gases trapped within the young Earth did not contribute to Earths later atmospheric composition. Heavy noble gases acquired during Earth’s formation contributed little to the evolution of Earth’s atmosphere. Noble gas isotopes are key tracers of both the origin of volatiles found within planets and the processes that control their eventual distribution between planetary interiors and atmospheres. Here, we report the discovery of primordial Kr in samples derived from Earth’s mantle and show it to be consistent with a meteorite or fractionated solar nebula source. The high-precision Kr and Xe isotope data together suggest that Earth’s interior acquired its volatiles from accretionary material similar to average carbonaceous chondrites and that the noble gases in Earth’s atmosphere and oceans are dominantly derived from later volatile capture rather than impact degassing or outgassing of the solid Earth during its main accretionary stage.

What CO2 well gases tell us about the origin of noble gases in the mantle and their relationship to the atmosphere

Study of commercially produced volcanic CO2 gas associated with the Colorado Plateau, USA, has revealed substantial new information about the noble gas isotopic composition and elemental abundance pattern of the mantle. Combined with published data from mid-ocean ridge basalts, it is now clear that the convecting mantle has a maximum 20Ne/22Ne isotopic composition, indistinguishable from that attributed to solar wind-implanted (SWI) neon in meteorites. This is distinct from the higher 20Ne/22Ne isotopic value expected for solar nebula gases. The non-radiogenic xenon isotopic composition of the well gases shows that 20 per cent of the mantle Xe is ‘solar-like’ in origin, but cannot resolve the small isotopic difference between the trapped meteorite ‘Q’-component and solar Xe. The mantle primordial 20Ne/132Xe is approximately 1400 and is comparable with the upper end of that observed in meteorites. Previous work using the terrestrial 129I–129Xe mass balance demands that almost 99 per cent of the Xe (and therefore other noble gases) has been lost from the accreting solids and that Pu–I closure age models have shown this to have occurred in the first ca 100 Ma of the Earths history. The highest concentrations of Q-Xe and solar wind-implanted (SWI)-Ne measured in meteorites allow for this loss and these high-abundance samples have a Ne/Xe ratio range compatible with the ‘recycled-air-corrected’ terrestrial mantle. These observations do not support models in which the terrestrial mantle acquired its volatiles from the primary capture of solar nebula gases and, in turn, strongly suggest that the primary terrestrial atmosphere, before isotopic fractionation, is most probably derived from degassed trapped volatiles in accreting material. By contrast, the non-radiogenic argon, krypton and 80 per cent of the xenon in the convecting mantle have the same isotopic composition and elemental abundance pattern as that found in seawater with a small sedimentary Kr and Xe admix. These mantle heavy noble gases are dominated by recycling of air dissolved in seawater back into the mantle. Numerical simulations suggest that plumes sampling the core–mantle boundary would be enriched in seawater-derived noble gases compared with the convecting mantle, and therefore have substantially lower 40Ar/36Ar. This is compatible with observation. The subduction process is not a complete barrier to volatile return to the mantle.

Application of Noble Gases to the Viability of CO2 Storage

Unequivocable evidence for warming of the climate system is a reality. An important factor for reducing this warming is mitigation of anthropogenic CO2 in the atmosphere. This requires us to engineer technologies for capture of our carbon emissions and identify reservoirs for storing these captured emissions. This chapter reviews advances made in understanding multiphase interactions and processes operating in a variety of subsurface reservoirs using noble gases. We begin by discussing the types of reservoir available for carbon storage and the mechanisms of viable CO2 storage, before summarising the physical chemistry involved in data interpretation and the sampling/sample storage techniques and requirements critical to successful sample collection. Theory of noble gas partitioning is interspersed with examples from a variety reservoirs to aid our knowledge of long term CO2 storage in the subsurface. These include hydrocarbon reservoir and natural CO2 reservoirs. In these examples we show how good progress has been made in using noble gases to explain the fate of CO2 in the subsurface, to quantify the extent of groundwater interaction and to understand CO2 behaviour after injection into oil fields for enhanced oil recovery. We also present recent work using noble gases for monitoring of subsurface CO2 migration and leakage in CO2 rich soils, CO2 rich springs and groundwaters. Noble gases are chemically inert, persistent and environmentally safe and they have the potential to be extremely useful in tracing migration of CO2. It is imperative that the many upcoming pilot CO2 injection studies continue to investigate the behaviour of noble gases in the subsurface and develop suitable noble gas monitoring strategies.

Predicting El Niño's Impacts

Insights into El Niños spatial structure may help to predict its effect on Atlantic tropical cyclones. The quasi-periodic cycle of warming and cooling in the eastern, near-equatorial Pacific Ocean known as the El Niño Southern Oscillation (ENSO) is associated with marked ocean temperature changes in the tropics and with long-range weather connections across the globe. In western South America, the cool phase (La Niña) brings dry conditions and excellent fishing in the nutrient-rich upwelling water, whereas the warm phase (El Niño) leads to floods and cutbacks in the fishing industry (1). But the notoriety of ENSO lies in its impact on seasonal weather around the globe, from droughts in Australia, to changes in the Indian summer monsoon and global tropical cyclone activity. In a landmark report on page 77 of this issue (2), Kim et al. revisit the structure of ENSO. The study has important consequences for the predictability of global weather patterns.